This section provides background information related to the present disclosure, which is not necessarily prior art.
Vacuum absorbing blow molded bases for use in hot-filled containers, such as polyethylene terephthalate (PET) containers, can be formed by over-stroke and counter-stretch blow-molding operations to create lighter weight base geometry with reduced average wall thickness as compared to conventional PET container bases. The over-stroke and counter-stretch blow-molding operations also allow for more aggressive geometric features to be formed, such as a deeper upstanding internal base wall and vacuum absorbing diaphragms with improved material distribution, stretch, and crystallization properties. The process facilitates fine material distribution control in both the axial and radial orientation directions, counter to the natural stretching and orientation behaviors of bottle grade PET polymeric materials.
Because stretching is fully controlled, it can be accomplished over the broadest range of temperatures. Higher temperatures reduce flexural modulus and molecular entanglement, and therefore require less pressure force to extend the molecular networks via the strain hardening mechanism. Orientation at the highest feasible temperatures, i.e., at the threshold of, or just below, 130° C. where the kinetics of thermally induced crystallization begin to dominate, are conducive to generation of fine crystalline entanglements. Orientation over the ideal range of ratios and temperatures provides resistance to retraction and distortion, imparting both transparency and thermal stability when re-exposed to temperatures above the Glass Transition Temperature of PET (Tg nominally=77° C.-80° C. for bottle grade PET resin grades suitable for heat set processing) during hot filling processes. One can anticipate achieving crystallinity levels (as measured by changes in density, infrared absorption, or Wide Angle X-Ray Scattering) on the order of 3%-5% higher than are achievable without the enhanced, fine material distribution and orientation control imparted by the use of a Counter-Stretching mechanism.
Therefore, a system that extends the range of utility of counter-stretching and over-stroking mechanisms has broad application to hot fillable packaging designs across the greatest range of sizes and processing conditions. Over-stroke can be defined by the mold components and process that facilitates the base geometry of a container being formed initially with the base component retracted from the mold, and a final step that moves the base component into the mold. The initial step forms the desired material properties such as surface area, thickness, and crystallinity. The final step forms the base shape and geometry of the final container.
Counter-Stretch can be defined by the mold components and process required to actuate a guide rod into the mold cavity and contact the external underside surface of a preform. The preform is captured between a stretch rod inside the preform and the counter-stretch rod as it is stretched and guided to its final position at the bottom of the container.
The over-stroke and counter-stretch operations can be combined into a single unit including two cylinders that control movement of the blow mold base, which is mounted to a top of the over-stroke unit. This unit is sized for specific ranges of container height and diameter, and for each blow mold there is a corresponding over-stroke unit. Thus multiple over-stroke units of different sizes are required to make containers of different sizes and shapes, which increases production costs and complicates production. The present teachings address these issues by providing over-stroke/counter-stretch assemblies that can be used to make containers of various different sizes and shapes.
With reference to FIG. 1 for example, over-stroke base unit 1010 is mounted to a rotary blow molding machine component called the base assembly 1012, which is attached to a casting called the console 1014. The corresponding blow mold 1016 is attached to a mold hanger 1018 that is mounted to the console 1014 directly above the base assembly 1012. The base assembly 1012 is moved vertically upwards into place and the mold 1016 closes for blow molding of a container to occur. When the container is completely formed, the base assembly 1012 retracts for clearance, the mold 1016 opens, and the finished container is removed from the blow mold 1016. The vertical upward and downward movement is controlled by a cam on the blow molding machine and a spring mechanism on the base assembly 1012. The functions of the cam and spring may be reversed depending on the type of blow molding machine being used.
When the base assembly 1012 is in the upward position and the mold 1016 closes, central counter-stretch cylinder actuates a rod 1020 into the mold to help center and guide the external tip of a preform during the blow molding process. When the base assembly 1012 is in the upward position and the mold 1016 closes, the body and base of the container are stretched and formed beyond the final height of the container. At a precise moment in time, the over-stroke cylinder actuates mold base 1022 (attached to the over-stroke unit 1010) and forces the base material into the final shape and container height. When the container is fully formed with high pressure air, and air in the container exhausted, the counter-stretch-rod 1020 and the over-stroke base unit 1010 retract, then the base assembly retracts 1012 allowing the mold 1016 to open and the finished container is removed from the blow mold 1016.
This blow molding process generally includes the following steps: (1) heated preform is placed in blow mold; (2) base assembly is actuated (by cam or spring); (3) mold is closed; (4) counter-stretch cylinder actuates the counter-stretch rod; (5) stretch & blow process is initiated; (6) over-stroke cylinder actuates the base; (7) stretch and blow process is completed; (8) counter-stretch, over-stroke, and base assembly retract; (9) mold opens; and (10) finished container is removed from mold.
While the blow molding device and method described above is suitable for its intended purposes, it is subject to improvement. For example, a blow-molding device and method that provides for at least the following would be desirable: allows containers of an increased height to be produced, such as containers having a height of greater than 250 mm; allows containers of an increased base surface area to be produced, such as a surface area of greater than 110 cm2; improves distribution of inertia during actuation of base over-stroke and counter-stretch; and addresses the complex, heavy, and costly over-stroke and counter-stretch mechanism associated with each mold set.